A common means of distributing energy around the world is by the transmission of gas, usually natural gas, but in some areas of the world manufactured gasses are also transmitted for use in homes and factories. Gas is typically transmitted through underground pipelines having branches that extend into homes and other buildings for use in providing energy for space and water heating. Many thousands of miles of gas pipeline exists in virtually every major city of the world. Since gas is useable only because it is highly combustible, gas leakage is a serious concern. For this reason much effort has been made to provide instrumentation for detecting small amounts of gas so that leaks can be located to permit repairs.
A known and successful system for detecting small quantities of gas in the environment is by the use of absorption spectroscopy. By this technique, a light beam of a selected frequency that is highly absorbed by the particular gas for which the instrument is designed is passed through a sample of the gas. The rate of absorption of the light beam is used as an indicator of the level of concentration of the gas in the sample. A basic element of natural gas, and most manufactured gasses used for room and water heating around the world, is methane. By initiating a beam of light at a frequency that is highly absorbed by methane and passing the beam through a sample of gas the level of concentration of methane in the gas sample can be determined.
In order to improve the sensitivity of detecting low levels of concentration of gas by spectral absorption, it is necessary to pass the light beam through a relatively long pathway of gas sample. Stating it another way, as the length of the light beam passing through a sample is increased, the sensitivity of the instrument to detect very small levels of gasses increase.
It is easy to understand that if a beam is passed through a very long tube containing a sample of gas that the instruments requiring such a long tube would be extremely cumbersome and therefore not easily portable. To overcome this problem, others have devised systems wherein a beam of light is repeatedly reflected between opposed mirrors to thereby extend the length of exposure of the beam to a gas sample in a way that the size of the instrument can be substantially reduced. A typical absorption cell is an elongated cylinder in which mirrors are disposed at opposite ends and light is introduced into the cells through a hole in one of the mirrors. For background information relating to the use of optical devices that provide for multiple traverses of light within a test cell having opposed mirrors reference can be had to the article entitled “Long Optical Paths of Large Aperture” by J. White J. Opt. Soc. Am. Vol 32, p. 285-288, May 1942. Another example of background information on this subject is entitled “Off-Axis Paths in Spherical Mirror Interferometers” by Herriott et al. in Applied Optics Vol. 3 pages 523-526. A further article by Herriott et al. entitled “Folded Optical Delay Lines” is found in Applied Optics, Vol 4 p. 883-889, August 1965. Because of the early work by Herriott in the development of light absorption spectroscopy using a cell having opposed mirrors in which a light beam is repeatedly reflected, such instruments are frequently referred to as “Herriott cells.” The invention herein relates to improvements and innovations in the construction, operation and use of Herriott type cells for detecting a selected gas, such as methane. Particularly, the invention herein provides methods and systems for detecting and measuring the level of concentration of a preselected gas using an instrument that is more portable, rugged and sensitive than other instruments and systems currently available.
For further background information relating to the basic subject matter of the invention herein reference maybe had to the following previously issued United States patents and other publications:
Patent orReference No.InventorTitle3,253,226Herriott et al.Optical Maser Amplifier3,437,954Herriott et al.Optical Delay Line Devices3,550,039Herriott et al.Optical Delay System4,934,816Silver et al.Laser Absorption DetectionEnhancing Apparatus and Method5,002,351Wolfum et al.Fusion Splicer for OpticalFibers5,121,405NegusAlignment Control Systemfor Lasers5,291,265KebarbianOff-axis Cavity AbsorptionCell5,528,040LehmannRing-down CavitySpectroscopy Cell UsingContinuous Wave Excitationfor Trace Species Detection5,550,636Hagans et al.Self-tuning Method forMonitoring the Densityof a Gas Vapor ComponentUsing a Tunable Laser5,637,872TulipGas Detector5,946,095Henningsen et al.Natural Gas DetectionApparatus and MethodOperable in a MovingVehicle5,949,537Inman et al.In-line Cell forAbsorption Spectroscopy6,064,488Brand et al.Method and Apparatus forIn Situ Gas ConcentrationMeasurement6,157,033ChudnovskyLeak Detection SystemUS PUBDiekmannInfrared Optical Gas Sensor2002/0011568US PUBPilgrim et al.Wavelength Agile External2002/0015427Cavity Diode LaserUS PUBGutinTunable Diode Laser System,2002/0018496Apparatus and MethodUS PUBSchleyMethod and Device for2002/0040590Determining the GasProperties of aCombustible GasUS PUBWarburtonMethod and Apparatus for2001/0045119Determining Concentrationof a GasFR PUBRonge et al.Precede et Dispositif deTrace d'Impuretesdans un Echantillon degaz au Moyen d'uneDiode Laser a SemiconducteurFR PUBTakeuchi et al.Water Content AnalysisH3-260859Device Using SemiconductorLaser, Double WavelengthDifferential AbsorptionMethodFR PUBTakeuchi et al.Analyseur de Teneur en eauH5-99845Utilisant un Laser aSemiconducteurOther Publications:    “Folded Optical Delay Lines,” Herriott et al., Applied Optics August 1965.    “Laser Beams and Resonators,” Kogelnik et al., Applied Optics October 1966.    “Narrow Optical Interference Fringes for Certain Setup Conditions in Multipass Absorption Cells of the Herriott Type,” McManus et al., Applied Optics Mar. 1, 1990.    “Measurement of Water Vapor Pressure and Activity Using Infrared Diode Laser Absorption Spectroscopy”, S. A. Bone, P. G. Cummins, P. B. Davies, S. A. Johnson, Applied Spectroscopy, Vol 47, no 6, 1993.    “Diode-Laser Absorption Technique for Simultaneous Measurements of Multiple Gas Dynamic Parameters in High-speed Flows Containing Water Vapor”, M. P. Arroyo, S. Langlois, R. K. Hanson; Applied Optics, Vol 33, no 15, 1994.    “Diode Laser Measurements of H2O Line Intensities And Self-Broadening Coefficients in the 1,4-μm Region”, S. Langlois, T. P. Birbeck and R. K. Hanson; Journal of Molecular Spectroscopy, Vol 163, p 27-42, 1994.    “Absorption Measurements of Water Vapor Concentration, Temperature, and Line-shape Parameters Using a Tunable InGaAsP Diode Laser”, M. P. Arroyo and R. K. Hanson; Applied Optics, Vol 32, no 30, 1993.    “Infrared Diode Laser Determination of Trace Moisture in Gasses”, J. A. Mucha, L. C. Barbalas, ISA Transactions, Vol 25, no 3, 1986.    “Application of Tunable Diode Lasers in Control of High Pure Material Technologies”, G. G. Devyatykhh, V. A. Khorshevh, G. A. Maksimovh, A. I. NadezhdinskiiA, S. M. Shapinh, Preprint.     “Laser Absorption IR Spectrometer for Molecular Analysis of High Purity Volatile Substances. Detection of Trace Water Concentrations in Oxygen Argon and Monogermane”, G. G. Devyatykh, G. A. Maksimov, A. I. Nadezhdinskii, V. A. Khorshev, S. H. Shapin; SPIE Vol 1724 “Turnable Diode Laser Applications”.    “Application of FM Spectroscopy in Atmospheric Trace Gas Monitoring: A Study of Some Factors Influencing the Instrument Design”, P. Werle, K. Josek and F. Slemr, SPIE Vol 1433 “Measurement Of Atmospheric Gases”, 1991.    “Stable Isotope Analysis using Tunable Diode Laser Spectroscopy”, Joseph F. Becker, Todd B. Sauke and Max. Loewenstein, Applied Optics, Vol 31, no 12, 1992.    “High Sensitivity Detection of Trace Gases using Sweep Integration and Tunable Diode Lasers”, D. T. Cassidy and J. Reid, Applied Optics, Vol 21, no 14, 1982.    “Atmospheric Pressure Monitoring of Trace Gases using Tunable Diode Lasers”, D. T. Cassidy and J. Reid, Applied Optics, Vol 21, no 7, 1982.    “Near Infrared Diode Lasers Measure Greenhouse Gases”, A. Stanton, C. Hovde, Laser Focus World, August 1992.    “Airborne Measurements of Humidity Using A Single Mode Pb Salt Diode Laser”, Joel A. Silver and Alan C. Stanton, Applied Optics, Vol 26, no 13, 1987.    “Diode Laser Spectroscopy for On Line Chemical Analysis”; David S. Bomse, David C. Hovde, Daniel B. Oh, Joel A. Silver and Alan C. Stanton, SPIE Vol 1681, “Optically Based Method for Process Analysis”, 1992.    “Two-mirror Multipass Absorption Cell”, J. Altmann, R. Baumgart and C. Weitkamp; Applied Optics, Vol 20, no 6, 1981.    “Long Optical Paths of Large Aperture”, J. White; J. Opt. Soc. Am. Vol 32, p 285-288, May 1942.    “Folded Optical Delay Lines”, Herriott et al.; Applied Optics, Vol 4 p 883-889, August 1965.    “Off Axis Paths in Spherical Mirror Interferometers”, D. Herriott, H. Kogelnik, R. Komper; Applied Optics, Vol 3 no 4, 1964.